Piezoelectric-electroluminescent display device containing nonlinear resistance layer



April 23, 1968 s. YANDO 3,379,927

7 PIEZOELECTRIC ELECTROLUMINESCENT DISPLAY DEVICE CONTAINING NONLINEAR RESISTANCE LAYER Filed Sept. 2. 1964 CURRENT OENSITY- AMPS/SQ. 0N.

' I0 I00 500 INVENTOR.

VOLTAGE STEPHEN YANDO Fig. 3.

A T TORNE X United States Patent 3,379,927 PIEZQELECTRlC-ELECTRQL UMINESQENT DlS- PLAY DEVHCE CONTATNING NONLINEAR Rn- SISTANE LAYER Stephen Yando, Hunfington, N.Y., assignor to General Telephone and Electronics Laboratories, 1110., a corporation of Delaware Filed Sept. 2, 1964, Ser. No. 393,953 Claims. (Cl. 3l5-55) ABSTRACT OF THE DISCLQSURE An electroluminescent display device in which a piezoelectric sheet is electrically coupled by a thin film to one surface of a nonlinear resistance layer and an electroluminescent layer is afiixed to the other surface of the resistance layer. Input signals are coupled to the piezoelectric sheet by electrodes secured to the edge thereof. The resulting display exhibits a high contrast ratio as compared to previous devices of this type.

This invention relates to display devices.

In my U.S. Patent 3,132,276 issued May 5, 1964, there is disclosed an electroluminescent device for visually displaying applied input signals as discrete points on an electroluminescent panel. In one embodiment of this invention, a rectangular electroluminescent phosphor layer is aflixed to one surface of a rectangular sheet of piezoelectric material. The piezoelectric sheet is provided with first and third parallel edges and second and fourth parallel edges, the second and fourth parallel edges being located between and perpendicular to the first and third edges. The surface area of the electroluminescent layer is less than that of the piezoelectric sheet and its sides are parallel to the edges of the sheet. A transparent conductive layer is placed over the electroluminescent layer while a common grounded conductive layer is secured to the other side of the piezoelectric sheet.

First, second, and third electrodes are secured to the piezoelectric sheet between its first, second, and third edges respectively and the electroluminescent layer. Thus, the first and third electrodes are parallel to each other and adjacent to opposite sides of the electroluminescent layer while the second electrode is perpendicular to the first and third electrodes. All four edges of the sheet are provided with terminations which absorb, substantially without reflection, any incident elastic waves.

A first voltage pulse applied between the first electrode and the grounded layer produces a mechanical strain in the piezoelectric sheet proportional to the amplitude of the first pulse. As the strain changes, a disturbance in the form of a first plane elastic wave accompanied by a first electric field is propagated from the first electrode toward the opposite edge of the sheet where it is absorbed by the termination. The intensity of the electric field is proportional to the time rate of change of the strain that produced it; i.e. the intensity of the first electric field is proportional to the first time derivative of the first pulse.

Similarly, second and third voltage pulses applied between the second and third electrodes respectively and the grounded layer produce second and third plane elastic waves. Each of these waves is accompanied by an electric field which is propagated from the corresponding electrode toward the opposite edge of the sheet where it is absorbed. The intensity of the second electric field is proportional to the first time derivative of the second voltage pulse and the intensity of the third electric field is proportional to the first time derivative of the third voltage pulse.

By applying voltage pulses to all three electrodes simultaneously, three elastic waves are generated which intersect in a small region near the center of the square electroluminescent layer. The intensities of the three electric fields, produced by the three waves are additive, resulting in a bright spot of light at the center of the layer. By adjusting the relative timing of the voltage pulses applied to each of the three electrodes, any selected area of the electroluminescent layer may be caused to glow. Further by continuously varying the relative timing of the three pulses, the spot of light can be caused to scan the panel at a velocity determined by the timing of the applied pulses.

In another form of the display device, a fourth electrode is secured to the piezoelectric sheet between the fourth edge of the sheet and the electroluminescent layer. A voltage pulse applied between this electrode and ground produces a fourth elastic wave accompanied by an electric field which propagates from the fourth electrode toward the opposite edge of the sheet where it is absorbed. By applying the voltage pulse to the fourth electrode at the correct instant of time relative to the time of application of pulses to the other three electrodes, the four waves are caused to intersect upon the selected point resulting in a bright spot of light in the electroluminescent layer at this point. Since the electric fields are additive, the use of four waves instead of three results in a brighter and more intense image.

Regardless of how many electrodes are used the brightest spot on the electroluminescent layer is always located at the point where all of the waves simultaneously intersect since it is at this point that the greatest voltage is applied across the layer. However, depending upon the magnitude of the applied voltage and the type of electroluminescent phosphor employed, secondary light outputs may also be produced in the electroluminescent layer wherever two of the waves intersect. This secondary light creates a background illumination which tends to reduce the contrast ratio; ie the ratio of the light emitted at a selected point on the electroluminescent layer to the light emitted by the remainder of the electroluminescent layer during the time required for an elastic wave to traverse the layer.

In order to improve the contrast ratio, a voltage-responsive nonlinear resistance layer may be inserted in series with the electroluminescent layer and piezoelectric sheet. As disclosed in my US. Patent 3,072,821, this results in the suppression of the secondary light output.

The nonlinear resistance layer is of the type wherein the resistance decreases as the voltage applied across the element increases. Stated another way, the current through the element varies according to the equation I=kV where l is the current through the nonlinear element, V is the voltage across it and i is a number greater than 1. Means are provided for exciting the piezoelectric element thereby generating a voltage across the nonlinear resistance element and across the electroluminescent layer. The larger the magnitude of this voltage, the lower is the resistance of the nonlinear element and, therefore, the larger is the percentage of the total voltage that appears across the electroluminescent a layer. Also, since the electroluminescent layer is essentially capacitive, the voltage across the layer reaches a maximum at a rate deter-mined by the magnitude of the charging current through the nonlinear element.

Although the nonlinear resistance layer efiectively suppresses the spurious light output of the electroluminescent layer, it has been found that it also imposes a mechanical restraint on the elastic wave resulting in the dispersion of the wave energy within the piezoelectric sheet. As a result, the resolution and brightness of the display is somewhat less than desired. Also, the total area of the display is limited by attenuation of the voltage pulses as they pass across the piezoelectric sheet.

Accordingly, it is an object of the present invention to provide an electroluminescent display device in which there is practically no background illumination when compared with the brightness of the image and which provides greater image brightness and improved resolution than prior devices.

Another object is to provide a display device employing electroluminescent and piezoelectric components which has a high contrast ratio but is not limited in size by attenuation of the voltage pulses.

Still another object is to provide an electroluminescent display device in which suppression of background illumination is obtained without undue reduction of the efliciency of the device.

Yet another object is to provide a means for efficiently coupling a piezoelectric sheet to relatively thick layertype elements,

A further object is to provide a display device in which the electroluminescent layer is coupled to a nonlinear resistance element with maximum efiiciency.

In accordance with the present invention, there is provided a display device in which a piezoelectric sheet is electrically coupled by a thin film to a relatively thick layer-type element composed of discrete particles having appreciable mass. The film provides electrical coupling between the two components while simultaneously mechanically decoupling them to prevent the dispersion of the elastic wave energy in the piezoelectric sheet. The coupling film has a relatively low impedance in a direction normal to the surface of the piezoelectric element and an impedance which is high in a direction parallel to the surface of the sheet. Further, the film has a shear modulus of elasticity which is as low as possible. The film is also sufiiciently resilient to bridge irregularities in themating surfaces when subjected to moderate pressures.

In one embodiment of the invention, a sheet of piezoelectric material is coupled to one surface of a nonlinear resistance layer by a thin conducting film. An electroluminescent layer is coupled to the other surface of the nonlinear resistance layer and the entire assembly interposed between a pair of conductors. In some cases improved electrical coupling can be obtained between the piezoelectric sheet and the nonlinear resistance layer by applying a matrix of discrete, closely spaced electrodes to the nonlinear layer at the interface between the layer and the piezoelectric sheet. The film may also be used for providing good electrical coupling between a piezoelectric sheet and other types of conductive and semiconductive layers such as ferroelectric sheet.

The above objects of and the brief introduction to the present invention will be more fully understood and further objects and advantages will become apparent from a study of the following description in connection with the drawings, wherein:

FIG. 1 is a cutaway schematic perspective view of one embodiment of a display device utilizing the inventtion;

FIG. 2 is a cross-sectional view of the display device of FIG. 1;

FIG. 3 is a log-log plot depicting the instantaneous current density-voltage characteristic of a typical nonlinear resistance layer used in the device of FIG. 1, and

FIG. 4 is a diagram showing, in detail, the coupling between the layers. v

Referring to FIGS. 1 and 2 there is shown a thin, square, polarized ceramic piezoelectric sheet 10 comprising a lead titanate-lead zirconate mixture. One surface of a nonlinear resistance layer 11 is electrically coupled to one surface of the piezoelectric sheet 10 by a conducting film 12 having a low shear modulus of elasticity, An electroluminescent layer 13 is coupled to the other surface of nonlinear resistance layer 12 by an intermediate electrode matrix 14. A transparent conductive electrode 15 is placed in intimate contact with electroluminescent layer 13 and a ground plane electrode 16 is secured to the other surface of piezoelectric sheet 19.

The nonlinear resistance layer 11 may be of the type disclosed in my U.S. Patent 3,220,881 granted Nov. 30, 1965 which consists of an essentially non-photoconductive cadmium sulfide powder embedded in an epoxy resin. FIG. 3 is a typical curve showing the current density in the nonlinear resistance layer when a voltage step is applied across the layer.

A first electrode 19 extending across the entire width of the electroluminescent layer 13 is secured to the piezoelectric sheet adjacent one edge of nonlinear resisance layer 11 while second, third, and fourth electrodes 2t 21 and 22 are secured to the piezoelectric sheet 10 adjacent the other edges of layer 11. While four electrodes have been illustrated, it shall be understood that a piezoelectric sheet having any number of electrodes attached thereto may be used.

Grounded electrodes 19a (not shown), 20a, 21a (not shown), and 22a are secured to the bottom of the piezoelectric sheet below corresponding top electrodes 19, 20, 21, and 22 and are electrically connected to ground plane electrode 16. Transparent conductive electrode 15 may be connected directly to the ground plane electrode 16 or a source of modulating voltage 25 may be coupled across electrodes 15 and 16 by means of a switch 26. Each edge of the piezoelectric sheet 10 is terminated in such manner as to absorb, substantially without reflection, any incident elastic wave propagated in the sheet. This is accomplished by coating the edges and immediately adjacent portions of sheet 10 with lead to provide terminations 30, 31, 32 and 33.

The application of a voltage pulse between electrode 19 and grounded electrode 19a causes a first elastic wave to be propagated across the piezoelectric sheet 10 at constant speed toward absorbing termination 32. This wave is accompanied by an electric field having an intensity proportional to the time rate of change of the pulse applied to electrode 19. A reverse wave also emanates from electrode 19 but is absorbed by termination 30 without affecting the display. Similarly, voltage pulses applied to electrodes 20, 21 and 22 cause second, third, and fourth elastic waves, accompanied by corresponding electric fields, to be propagated at the same speed as the first wave toward terminations 33, 30 and 31 respectively.

As each of the waves emanating from adjacent per pendicular electrodes intersect, a pulse voltage having an amplitude equal to 2 v. is applied across the electroluminescent and nonlinear resistance layers in series. These voltages occur along diagonal lines forming the loci of the intersections of the waves from perpendicular pairs of electrodes 1? and 29, 20 and 21, 21 and 22, and 22 and 19. In addition, voltages equal to 2 v. appear across the nonlinear and electroluminescent layers along vertical and horizontal lines formed by the intersection of waves from opposite electrodes 19, 21 and 20, 22 respectively. However, due to the high resistance of the nonlinear resistance layer 11 the current charging the electroluminescent layer 13 at each point of intersection is quite low and therefore the voltage appearing across layer 13 is insufficient to produce visible light.

When voltage pulses are applied to the four electrodes 19-22 simultaneously, the four elastic waves meet at the center of the electroluminescent layer 13. By applying voltage pulses to electrodes 19-22 at different instants of time, the four valves can be made to intersect at any selected point on the electroluminescent layer 13. At the point of intersection of the four waves, the voltage across the electroluminescent and nonlinear layers is equal to 4 v. Almost all of this voltage is applied initially across nonlinear resistance layer 11 driving its resistance down and causing a large charging current to flow into the portion of electroluminescent layer 13 at the point of intersection. The large charging current produces a rapid increase in the voltage across layer 13 and, as a result, a relatively intense spot of light is emitted by the electroluminescent layer at the intersection of the four elastic waves. The light emitted from the electroluminescent layer 13 at the points where the voltage is 2 v. is insignificant when compared to the intensity of the image at the intersection where the applied voltage equals 4 v. As shown in the log-log plot of FIG. 3, the current density in a typical nonlinear resistance layer is approximately twenty five times as great when a voltage step of 200 volts is applied to it as it is when a step of 100 volts is applied across the layer.

It has been found, however, that a nonlinear resistance layer of sufiicient thickness and mass to provide good attenuation of spurious background illumination also imposes a mechanical restraint on the elastic waves propagated through the piezoelectric sheet 10. Conducting lrn 12 minimizes this mechanical restraint and also provides the necessary electrical coupling between piezoelectric sheet and nonlinear resistance layer 11.

A medium suitable for use as the coupling means 12 is glycerol given by the formula C H O This liquid, which provides primarily capacitive coupling, has a dielectric constant at 1000 cycles per second of 42.5 and a shear modulus having a value which is substantially equal to zero. A thin film of glycerol has a low impedance in a direction normal to the surfaces of piezoelectric sheet 10 and a high impedance in a direction parallel to the surfaces of sheet 10. By low impedance, it is meant that the impedance of the film is considerably less than the impedance of the load presented by the nonlinear resistance layer 11 and the electroluminescent layer 13. In the described device, the load impedance in series with the coupling film is essentially capacitive in nature having a capacitance of approximately 2000 micromicrofarads per square inch. A glycerol film having a capacitance of 20,000 micromicrofarads per square inch or greater has been found suitable for use in this device.

Alternatively, a film which is primarily resistive, such as a polysulphide gel containing a carbon filler, may be used. A coupling film of this type having a thickness of about 0.005 inch and a resistance of 2500 to 25,000 per cubic centimeter has been successfully employed.

In fabricating the device, the nonlinear resistance layer 11, electrode matrix 14, electroluminescent layer 13 and transparent conductive electrode 15 are first assembled in a suitable frame (not shown). Coupling film 12 is then formed by spreading several drops of glycerol on the piezoelectric sheet 19 and the surface of the nonlinear resistance layer 11 then pressed against sheet 10 to form a good electrical connection.

The action of the coupling film 12 in increasing the electrical coupling may be better understood from a study of FIG. 4 which shows an enlarged view of the particles of the nonlinear resistance layer 11. As shown, particles of layer 11 touch the piezoelectric sheet 10 at discrete points such as 40 thereby presenting a high resistance to current flow in the layer. In addition, the thickness and mass of the nonlinear resistance layer tend to act as a mechanical restraint on the piezoelectric sheet causing dispersion of the elastic wave. By interposing the glycerol layer 12 between sheet 10 and nonlinear resistance layer 11 the conductivity normal to the surface of the piezoelectric sheet 10 is increased since a greater area for current conduction is presented. In addition, since the shear modulus of elasticity of the glycerol is negligible, the mechanical restraint on piezoelectric sheet 10 is practically eliminated.

Another material which has been found satisfactory for use as the coupling film 12 is a polysulphide gel containing 10% acetylene black by weight. The gel found most satisfactory has a composition consisting of 5.0 parts liquid polysulphide polymer (Thiokol LP-Z), 10.0 parts orthonitrobiphenol plasticizer, 7.5 parts Arochlor No. 1262 plasticizer, 0.5 part cumene hydroperoxide catalyst and 0.2 part amine DNP-30 accelerator. This material has a resistivity of approximately 2500 ohms per cubic centimeter and a shear modulus of approximately one pound per square inch.

A matrix of electrodes 14 shown in FIGS. 2 and 4 improves the electrical coupling between the nonlinear resistance layer 11 and the electroluminescent layer 13. Because of the granular nature of the polycrystalline electroluminescent and nonlinear layers intimate contact would be obtained only at isolated points if the electrodes were omitted. Electrodes 14 are subdivided into a matrix of elements electrically isolated from each other to limit the display signal to a small area of electroluminescent layer 13. To avoid loss of resolution, the size of these electrode elements is made small compared to the trace width.

When switch 26 is thrown to connect the modulation generator 25 across electrodes 15 and 16, an additional voltage is superimposed on that produced by the elastic waves traversing the piezoelectric sheet 10. As before, the nonlinear resistance layer serves to reduce all background light to insignificant proportions and the conductive film 12 improves the electrical coupling between the piezoelectric sheet 10 and nonlinear resistance layer 11 while minimizing the mechanical restraint in the piezoelectric sheet.

As many changes could be made in the above construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A display device comprising (a) an electrically nonlinear resistance layer having first and second surfaces,

(b) a sheet of piezoelectric material,

(0) a coupling film interposed between the first surface of said nonlinear resistance layer and said sheet of piezoelectric material, said film having an electrical impedance in a direction normal to the surface of said piezoelectric sheet and a shear modulus of elasticity which are as low as possible,

(d) an electroluminescent layer coupled to the other surface of said nonlinear resistance layer, and

(e) first and second conductors coupled to said electroluminescent layer and said sheet of piezoelectric material respectively.

2. A display device comprising (a) an electrically nonlinear resistance layer having first and second surfaces,

(b) a sheet of piezoelectric material,

(c) a coupling film interposed betweenthe first surface of said nonlinear resistance layer and said sheet of piezoelectric material, said film having an elecrical impedance between 2500 and 25,000 ohms per cubic centimeter and a shear modulus of elasticity not greater than about one,

(d) an electroluminescent layer coupled to the other surface of said nonlinear resistance layer, and

(e) first and second conductors coupled to said electroluminescent layer and said sheet of piezoelectric material respectively.

3. A display device comprising (a) an electrically nonlinear resistance layer having first and second surfaces,

(b) a sheet of piezoelectric material,

(c) a coupling film interposed between the first surface of said nonlinear resistance layer and said sheet of piezoelectric material, said film having an electrical capacitance greater than 20,000 micromicrofarads per cubic centimeter and a shear modulus of elasticity not greater than about one,

(d) an electroluminescent layer coupled to the other surface of said nonlinear resistance layer,

(e) first and second conductors coupled to said electroluminescent layer and said sheet of piezoelectric material respectively, and

(f) electrode means adapted to receive an applied voltage affixed to said piezoelectric sheet, an elastic wave accompanied by an electric field being propagated in said sheet of piezoelectric material upon excitation of said electrode means by said applied voltage.

4. A display device as defined by claim 3 wherein said coupling film is composed of glycerol.

5. A display device comprising (a) an electrically nonlinear resistance layer having first and second surfaces,

(b) a sheet of piezoelectric material,

(c) a coupling film interposed between the first surface of said nonlinear resistance layer and said sheet of piezoelectric material, said film having an electrical impedance in a direction normal to the surface of said piezoelectric sheet and a shear modulus of elasticity which are as low as possible,

((1) an electroluminescent layer,

(e) a matrix consisting of a pluralityof electrically isolated electrodes interposed between said electroluminescent layer and the second surface of said nonlinear resistance layer,

(f) first and second conductors coupled to said electroluminescent layer and said sheet of piezoelectric material respectively, and

(g) a plurality of electrode pairs secured to the surfaces of said piezoelectric sheet, an elastic wave accompanied by an electric field being propagated in said sheet when a voltage is applied across an electrode pair.

6. A display device comprising (a) an electrically nonlinear resistance layer having first and second surfaces,

(b) a sheet of piezoelectric material having first and second surfaces,

(c) a coupling film interposed between the first surface of said nonlinear resistance layer and the first surface of said sheet of piezoelectric material, said film having an electrical capacitance greater than 20,000 micromicrofarads per cubic centimeter and a shear modulus of elasticity not greater than about one,

(d) an electroluminescent layer having first and second surfaces,

(e) a matrix consisting of a plurality of electrically isolated electrodes interposed between said first surface of said electroluminescent layer and the second surface of said nonlinear resistance layer,

(f) a transparent conductive layer affixed to the second surface of said electroluminescent layer,

(g) a ground plane electrode secured to the second surface of said sheet of piezoelectric material, and

(h) a plurality of electrode pairs secured to the surfaces of said piezoelectric sheet, an elastic wave accompanied by an electric field being propagated in said sheet when a voltage is applied across an electrode pair.

7. A display device comprising (a) a rectangular electrically nonlinear resistance layer having first and second surfaces,

(b) a rectangular sheet of piezoelectric material having first and second surfaces, the area of said sheet being greater than that of said nonlinear resistance layer,

(c) a coupling film interposed between the first surface of said nonlinear resistance layer and the first surface of said sheet of piezoelectric material, said film having an electrical impedance between 2500 and 25,000 ohms per cubic centimeter and a shear modulus of elasticity not greater than about one,

(d) a rectangular electroluminescent layer having first and second surfaces deposited in registration with said nonlinear resistance layer,

(e) a matrix consisting of a plurality of electrically isolated electrodes interposed between said first surface of said electroluminescent layer and the second surface of said nonlinear resistance layer,

(f) a transparent conductive layer affixed to the second surface of said electroluminescent layer,

(g) a ground plane electrode secured to the second surface of said sheet of piezoelectric material, and

(h) first and second pairs of electrodes secured to said piezoelectric sheet adjacent said nonlinear resistance layer, said first and second pairs of electrodes being located along perpendicular edges of said rectangular sheet, elastic waves accompanied by corresponding electric fields being propagated in said sheet when voltages are applied across said electrode pairs.

8. A display device comprising (a) a reactangular electrically nonlinear resistance layer having first and second surfaces,

(1)) a rectangular sheet of piezoelectric material having first and second surfaces, the area of said sheet being greater than that of said nonlinear resistance layer,

(c) a coupling film interposed between the first surface of said nonlinear resistance layer and said sheet of piezoelectric material, said film having an electrical impedance in a direction normal to the surface of said piezoelectric sheet and a shear modulus of elasticity which are as low as possible,

(d) a rectangular electroluminescent layer having first and second surfaces deposited in registration with said nonlinear resistance layer,

(e) a matrix consisting of a plurality of electrically isolated electrodes interposed between said first surface of said electroluminescent layer and the second surface of said nonlinear resistance layer,

(f) a transparent conductive layer affixed to the second surface of said electroluminescent layer,

g) a ground plane electrode secured to the second surface of said sheet of piezoelectric material, and

(h) first, second, third and fourth pairs of electrodes secured to said piezoelectric sheet adjacent said nonlinear resistance layer, each of said pairs of electrodes being located along a corresponding edge of said rectangular piezoelectric sheet, elastic waves accompanied by corresponding electric fields being propagated in said sheet when voltages are applied across each of said electrode pairs.

9. In an electroluminescent device,

(a) a pair of spaced conductors,

(b) a sheet of piezoelectric material having first and second surfaces, the second surface of said sheet being coupled to one of said spaced conductors,

(c) an electrically nonlinear resistance layer having first and second surfaces, the second surface of said nonlinear layer being coupled to the other of said pairs of spaced conductors, and

(d) a coupling film interposed between the first surface of said nonlinear layer and said sheet of piezoelectric material, said film having an electrical impedance in a direction normal to the surface of said piezoelectric sheet and a shear modulus of elasticity which are as low as possible.

10. In an electroluminescent device,

9 10 (a) apair of spaced conductors, piezoelectric sheet and a shear modulus of elas- (b) a sheet of piezoelectric material having first and ticity Which are as low as Possible, and

Second urfa h second surface f Said Sheet (e) an electroluminescent layer interposed between the second surface of said nonlinear layer and the other being coupled to one of said spaced conductors,

of sa1d pair of spaced conductors.

(c) an electrically nonlinear resistance layer having 5 first and second surfaces, No references cited.

a coupling interPosed betwfien the JAMES W. LAWRENCE, Primary Examiner.

face of sa1d nonllnear layer and said sheet of piezoelectric material, said film having an electrical irn- 10 DAVID GALVIN pedance in a direction normal to the Surface of said RO R UDD, Assistant Examiner. 

